Precision stages are becoming increasingly important in industrial applications, such as semiconductor fabrication, precision manufacturing, machine tools, lithography, and scanning probe microscopes. Stage accuracy to the micrometer and even nanometer scale is quickly becoming the standard in a number of the above-mentioned industries. To overcome the friction inherent in conventional stage designs involving rotation motors and lead screws, linear motors are increasingly being used as stage drive mechanisms. Additionally, advances in technology have led to an interest in planar stages for replacing conventional two-layer biaxial systems in order to minimize friction, spatial requirements, and other unwanted effects such as Abbe error (also called sine error, which indicates the magnification of an angular error over a distance).
Use of linear motors, however, introduces thrust ripple that arises from unwanted variations in an input current, a cogging force, or imperfections in the coils within the motors and other sources. Recent research has aimed to identify and overcome thrust ripple in permanent magnet linear synchronous motors (PMLSMs) and similar hardware through development of such techniques as an adaptive sliding control for thrust ripple compensation, the application of a position-dependent compensation term in the control algorithm, improvement in motor hardware design and magnetic field analysis using the equivalent magnetizing current method, the application of repetitive control, and variations of additional hardware parameters such as magnet size.
However, the above-mentioned research requires a system and method to measure thrust ripple precisely in a two-axis precision stage so as to achieve thrust ripple compensation.